550 5.7.520 error: external forwarding not allowed
Microsoft's 550 5.7.520 isn't a spam block: the forwarding mailbox's own organization blocks automatic forwarding to outside addresses.
What 550 5.7.520 means
550 5.7.520 Access denied, Your organization does not allow external forwarding.An outbound spam filter policy blocks automatic forwarding to external addresses in that organization.
List problem or sender problem?
Their mailbox. It's an admin policy in the Microsoft 365 organization doing the forwarding. If it came back from a prospect, their mailbox tried to forward your email outside and their organization stopped it. It's a permanent failure: the server won't accept this message if you send it again unchanged.
How to fix 550 5.7.520
- If it's your own organization, ask your admin to review the outbound policy for external forwarding, or stop auto-forwarding outside.
- If it came back from a prospect's side, there's nothing to fix on yours. Reach them another way.
In SendingOS, 550 5.7.520 shows on the conversation with its reason and code, so a problem on their side never gets mistaken for one on yours.
Not sure whether a spike in bounces is the list or the domain? Read High email bounce rate? Burned domain or bad list.
Related codes
Sources: Microsoft: NDRs in Exchange Online